Reduces the mana cost and increases your chance to hit with spells by X%.
Precision is a mage talent that increases your chance to hit and reduces mana cost of all spells by 1% per rank, up to 3% at max rank 5 in patch 3.0.2 when it was changed to hit rate instead of resistance. At level 80, each point is equivalent to 12.6 spell hit rating. In PvE raid situations, many mobs and especially bosses are up to three levels higher than the mage, increasing resist rate, thus making this talent very popular with raiding mages. The additional mana efficiency is also desirable for PvE.
Normally a talent that does not scale with gear upgrades is not desirable. Although Precision is still a useful talent, because one needs static 17% to hit to ensure all spells will hit. Taking this talent means a mage can reach the spell hit cap sooner, focusing gear in other areas. The talent still scales appropriately with level, as well.
See the article on Resistance for more information on the resistance chance of higher-level enemies.